1. Field of the Invention
This invention relates to a high strength stainless steel suitable for use in components requiring high strength and corrosion resistance in office machines, electrical communication equipments, measurement instruments, automobile parts and the like, such as thin leaf spring, coil spring, antenna, precision thread and so on. More particularly, it relates to high strength stainless steels having a tensile strength of not less than 230 kgf/mm.sup.2, which has never been attained in the conventional precipitation hardened stainless steel, through ageing treatment after cold working.
2. Related Art Statement
Heretofore, JIS SUS 301 (0.1%C-17%Cr-7%Ni-Fe) after cold working and SUS 631 (0.07%C-17%Cr-7%Ni-1%Al-Fe) after cold working and ageing are frequently used as a spring material for office machines, electrical communication equipements and the like in view of the corrosion resistance. These stainless steels have strengths of about 190 kgf/mm.sup.2 and 210 kgf/mm.sup.2 at maximum, respectively. Lately, it is demanded to develop stainless steels for spring having a strength of not less than 230 kgf/mm.sup.2 with the tendency of the miniaturization, weight reduction and high performance in the office machines, electrical communication equipments and the like.
In general, however, as the strength of the stainless steel for spring becomes higher, the toughness and ductility become lower, so that it is difficult to form the spring from such a steel by means of a press machine, a coiling machine or the like. Particularly, when the strength exceeds 200 kgf/mm.sup.2, there may be caused the breaking of the steel material during the formation of the spring. Therefore, if it is intended to provide the strength of not less than 200 kgf/mm.sup.2 at a use state, the steel material is first formed into a spring at such a state that the strength of the steel material is less than 200 kgf/mm.sup.2 in order to avoid the breaking of the steel material, and then the increase of the strength should be attained by any method.
Hitherto, metastable austenite-type precipitation hardened stainless steels represented by JIS SUS 631 and 15-7Mo steel (0.02%C-15%Cr-7%Ni-1.2%Al-2.3%Mo-Fe) have been used along the above requirement. This type of the stainless steel is at an austenite state after solution treatment and is drawn to a strength of not more than 200 kgf/mm.sup.2 capable of forming of the spring, during which austenite is transformed into martensite. At such a state, the steel is shaped into the spring of a given form, which is then hardened by an ageing treatment.
In the above conventional technique, however, the elemental amounts of Al, Mo and so on precipitated by the ageing treatment are small, so that the tensile strength after the ageing treatment is 220 kgf/mm.sup.2 at most.
In order to further increase the tensile strength at the use state, it is effective to increase precipitation hardening elements such as Al, Mo and so on, but as these elements become larger, austenite is stabilized and hardly transformed into martensite even by working.
In order to evaluate the stability of austenite, Md.sub.30 is used as an indication. This Md.sub.30 is defined by "temperature of transforming 50% of austenite into martensite under a true strain of 0.3". For instance, T. Angel proposes the following equation (1) as a relationship between Md.sub.30 and chemical composition of steel: EQU Md.sub.30 (.degree.C.)=413-462.times.[%C+%N]-9.2.times.[%Si]-8.1.times.[%Mn]-13.7.ti mes.[%Cr]-7.5.times.[%Ni]-18.5.times.[%Mo] (1).
According to the equation (1), for example, if the amount of Mo is increased, when the amounts of Cr and Ni are decreased at a rate corresponding to the decreased rate of Md.sub.30, the value of Md.sub.30 , the value of Md.sub.30 can be made unchangable. However, the decreases of Cr and Ni also reduce Ni equivalent and Cr equivalent calculated by the following equations (2) and (3): EQU Ni equivalent=[%Ni]+30.times.[%C]+0.5.times.[%Mn]+0.3.times.[%Cu](2) EQU Cr equivalent=[%Cr]+[%Mo]+1.5.times.[%Si]+0.5.times.[%Nb] (3)
so that the structure of the steel alloy is closed to martensite+ferrite phase as shown in Schaeffler diagram of FIG. 1. Therefore, the work hardening by the drawing is small, and particularly the hot workability is considerably deteriorated.
Thus, it is very important that in the metastable austenite-type stainless steel, phase transformation temperature is finely controlled for ensuring the stable quality.